Nicotine addiction is a major worldwide medical and social problem that continues to escalate. Affective and somatic symptoms associated with nicotine withdrawal contribute to the difficulty in quitting tobacco use. While current therapies for smoking cessation are helpful, none can claim a very high rate of success. Therefore, there is a great need for a better understanding of the behavioral and biological factors underlying nicotine withdrawal. Stress and anxiety play an important role in the maintenance of nicotine abuse. Anxiety promotes cigarette use and it is also one of the symptoms of withdrawal, making tobacco abstinence more difficult. This application builds on the observation that lack of (4* or [unreadable]5* nicotinic acetylcholine receptors (nAChRs) significantly reduces somatic signs of withdrawal and reduces anxiety-like responses. (4* and [unreadable]5* nAChRs are prominently expressed in the medial habenula/interpeduncu[unreadable]lar nucleus (MHb/IPN) axis. The habenula is a source of negative reward signals and participates in a wide array of behaviors including anxiety and stress. We postulate that (4* and [unreadable]5* nAChRs in the MHb/IPN axis are in a key position to influence the mesolimbic dopamine system during nicotine withdrawal. Control mice and mice lacking the (4 or the [unreadable]5 nAChR subunit will be chronically treated with nicotine delivered in the drinking water, and withdrawal will be precipitated by injection of the nAChR antagonist mecamylamine. First, we will examine the role of (4* and [unreadable]5* nAChRs on anxiety-related responses during withdrawal using a battery of behavioral paradigms. Second, we will ask which manifestations of nicotine withdrawal are directly influenced by the (4* and [unreadable]5* nAChRs expressed in the MHb/ IPN axis. In those experiments, lentiviral vectors will be used to either re-express (4 or [unreadable]5 in the MHb or the IPN of (4 and [unreadable]5 null mice or to knock down the levels of [unreadable]5 and [unreadable]4 in MHb and IPN of wild type mice.